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xen-devel
Re: [Xen-devel] PHY vs. VD benchmark
> I suspect you might be able to increase the performance by using a different
> extent size. The VD management code allocates extents of "initialised"
> physical disks. The extent size for a device can be specified when it's
> initialised. The default is 64 megabytes. A 4 gigabyte virtual disk would
> make for 64 extents of 64 meg. A long list of extents could be slowing down
> the disk address translation in Xen, resulting in poorer benchmark results.
> At some stage I might rethink the default extent size.
The correct fix is to get rid of the linear linked lists.
A good way to do this is to use a buddy allocator for allocating VDs:
you track power-of-two multiples of 64MB free disc space, and try to
create VDs out of the largest possible extents. Within Xen you can
then use a truncated radix tree to map virtual extents to real extents
--- Linux has code we can use for truncated radix trees.
-- Keir
> If you want, you could try a larger extent size by specifying it at
> initialisation time. [ you can't change the extent size on an already
> initialised device :-( ]. This might speed things up in the benchmark,
> although if you don't allocate space in multiples of the extent size there
> will be more space in unused "partial extents" than for a smaller extent size.
>
> In the degenerate case that your entire virtual disk fits inside an extent,
> it
> should give exactly the same performance as for the phy case (since the code
> will be doing exactly the same thing).
>
> If I get some spare time I might test this out myself - right now I'm
> debugging a different problem.
>
> Cheers,
>
> Mark
>
>
>
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